1. Field of the Invention
The present invention relates to a method for the co-deposition of silicon and nitrogen on a stainless steel surface, and more particularly to a method of deposition using silicon nitride powder as the source material by a pack cementation process.
2. Description of the Related Arts
It is known that the addition of proper amounts of silicon to stainless steel not only increases the resistance to oxidation at high temperature, but also elevates the hardness. Further, if the stainless steel contains more nitrogen, its resistance to corrosion, like pitting, will be improved as well. However, when using conventional metallurgical techniques to add silicon or nitrogen to stainless steel, such as smelting and casting followed by cold processing, the resulting casting will become brittle due to the high silicon content. Therefore, it is advantageous to modify the surface of a stainless steel work piece instead.
Conventional surface deposition techniques for alloying include pack cementation and laser scan. The pack cementation technique usually employs elemental silicon as source material. For example, U.S. Pat. No. 5,589,220 discloses a method for depositing silicon and chromium onto the surface of a metal using silicon and chromium powders as source materials and an activator. U.S. Pat. No. 5,492,727 also uses silicon and chromium powders as source materials, however, it uses at least two activators for depositing. Then, the surface alloy layer is formed in a diffusion manner. U.S. Pat. No. 5,364,659 uses chromium-silicon master alloy as source material, and uses a mixture of activators to produce a metal surface with a diffusion layer containing high silicon and chromium. U.S. Pat. No. 4,500,364 uses elemental aluminum and silicon or Al-Si eutectic or Al-Si hypereutectic as source material to produce the surface of a diffusion layer containing aluminum and silicon. USRE 029212 discloses a method for producing an aluminum cladded material. All the patents described above employ the pack cementation technique. The differences among them are the different source materials and activators used, and different contents in the resulting surface diffusion layer. None of the patents, however, discloses using silicon nitride as source material. Silicon nitride may be used as source material when using the laser scan technique for alloying. However, the device used in this method is expensive and the control of atmosphere in the process is also complex. Further, this method is not suitable for an article with complex shape. Thus, the applications of laser scan technique for alloying are limited. There are presently no references disclosing the depositing of nitrogen using the pack cementation technique.
It is therefore a primary object of the present invention to provide a method for co-depositing silicon and nitrogen coatings on a stainless steel surface, the method comprising the steps of: (a) cleaning the surface of said stainless steel by mechanical and/or physical and chemical means; (b) placing the stainless steel in a cementation pack including at least a mixture of sodium fluoride, silica and silicon nitride powder; and (c) heating said stainless steel and cementation pack in an inert atmosphere, wherein the silicon nitride powder is thereby decomposed into elemental silicon and nitrogen and diffusely coated onto the surface of said stainless steel to form a nitrogen-containing high silicon stainless steel.
Another object of the present invention is to provide a metal having a stainless steel surface on which silicon and nitrogen are deposited, wherein the surface is comprised of 1% to 15% silicon content and 0.1% to 0.6% nitrogen content, and the metal is produced by the method described above.